Construct safer Escherichia coli K-12 strains for DNA recombinant research by introducing mutations that will: increase the usefulness of strains for this research; preclude or minimize the ability of strains to colonize and/or survive in the intestinal tracts of animals; preclude biosynthesis of the bacterial cell wall and mucopolysaccharide capsular material in all environments other than in specially defined culture media; lead to degredation of genetic information at mammalian body temperatures; cause plasmid and/or phage cloning vectors to be dependent upon host provided functions; minimize transmission of recombinant DNA to other bacteria in nature; and permit monitoring.